(1) Field
The present invention relates to the field of signal processing and data communications. More specifically, the present invention relates to a method, apparatus, and system for improving power amplifier efficiency.
(2) General Background
In the past few years new, high technology communication products designed for consumers have offered lucrative business opportunities. As a result, significant efforts have been made towards development of technologies and products. Such products can include cellular phones, wireless local area networking cards, DSL cards, products providing Internet access through cable or satellite, etc.
Two important factors that contribute to the success of a consumer communication product are:
1. Cost of manufacturing. In the face of competition to reduce product prices, lower cost products have higher chance of success;
2. Power consumption of the end product. For wireless products such as a cellular phones or laptop computers, attractiveness of the product is very closely linked with the length of time the device can operate between battery recharges. In order to improve the battery life between recharges, it is necessary to reduce the power consumption of the product.
The power amplifier (PA) of a wireless transmitter is one of the system elements that is typically a stand alone component. In most applications, the PA module is purchased from a third party vendor and the cost of it is a significant contributor to the overall cost of the product. Also, power amplifiers typically consume significant power.
Given that a wireless transmitter should transmit at predetermined power levels, the portion of the power that a PA yields relative to the total power it absorbs from the battery is one of the key characteristics of a power amplifier. The ratio of PA output power relative to total power absorbed from the battery is called the efficiency of a power amplifier. In typical state of the art in PA systems, the efficiency of a power amplifier increases with increasing input signal power level. The maximum efficiency is reached at input levels that cause saturation of the power amplifier. The term saturation is used to describe the operating condition in which, the PA operation reached a point at which if the input level to the PA is increased, PA output signal level does not change, or, it changes slightly. In engineering jargon this is called “driving the PA to saturation”.
Unfortunately, many modern communication signal formats do not lend themselves be transmitted through a transmitter that is driven to saturation. Specifically, modulation signals that do not have a constant envelope are significantly affected when passed through a non-linear transmitter. There are two main effects of a non-linear system on a non-constant envelope signal:
1. Reduction in signal quality: The transmitted information is encoded both in the amplitude and phase of a transmitted signal, and, distorting the amplitude of the signal leads to loss of quality of the transmitted signal.
2. Production of extra spectral components outside the bandwidth of signal: These extra spectral components may result in interfering in adjacent frequency channels if they are not filtered successfully before transmission. Typically the amount of extra spectral component that a transmitter can transmit at an adjacent channel is strictly regulated by standards that a transmitter should adhere to for obtaining a license for a product to be sold.
Several commercial consumer communication products utilize non-constant envelope modulation formats due to their high spectral efficiency. For example 2G and 3G Code Division Multiple Access (CDMA) cellular phone and Orthogonal Frequency Division Multiplexing (OFDM) wireless local area network (LAN) signals have significantly varying envelopes.
In order to reduce distortion to such signals, typically the input drive level is a kept a few decibels (dBs) less than the level that would have driven the PA to saturation. In communication systems engineering terminology, this is called “operating the PA in back off.”
However, as we had mentioned earlier, the PA efficiency is low when it is driven at levels that are lower relative to saturation. This is one of the key problems with PA technology that prompted many studies and to which the present invention is directed to improve.
In efforts to improve efficiency of power amplifier following methods have been utilized:
1. PA bias level adaptation
2. Operating the PA with reduced back off. There are two alternative approaches:
                a. PA linearization        b. Signal conditioning to reduce effects of PA non-linearity on signal.PA Bias Level Adaptation        
Typically, wireless communication applications have multiple power level modes of operation. It is a common technique to modify the power amplifier bias current to appropriate values at each required output power level. The saturation level at the output of a PA is dependent on the bias current. If the bias current is increased, the PA linear range increases, and, the PA can provide higher output power levels without distortion. However at the same time, the power consumption of the PA increases with increasing bias current. Thus, it is advantageous to keep the bias current as small as possible. Thus, for each output power level desired, the bias current is reduced as much as possible. The level that bias current can be reduced is determined also by the desired back off that the PA should operate relative to saturation. The required PA back off is chosen so as to reduce the distortion to the transmitted signal, and, out of band spectral emissions to acceptable levels.
Reduced Back Off
In efforts to improve PA efficiency one of the key design approaches is generally to find methods to reduce the amount of PA back off. Thus, the drive level of the power amplifier increased so that the PA will operate closer to saturation. However, a key problem with this approach is that, as the operation point gets close to saturation, the distortion introduced by the PA nonlinear characteristic increases.
To reduce the effects of reduced PA back off two methods are used:
Reduced Back Off with PA Linearization
PA Linearization techniques are used for compensating the non-linearity of the PA characteristics. These essentially correct the distortion introduced by the PA by means of various methods.
Reduced Back Off with Signal Conditioning
For typical communication signals used in modern applications, the instantaneous power of the modulated signal can be significantly higher relative to the average power level. Typically the variation of modulated signal around its average value is measured by a parameter called Peak to Average Power Ratio (PAPR). More precisely, PAPR is the ratio of power of a hypothetical signal that has the same amplitude as the peak instantaneous power of the signal divided by the average power of the signal. The larger PAPR ratio a communication signal has, the larger should the back off be relative to saturation. This is because, the signal can have large variations and if the peak values drive the PA into saturation, the distortions are introduced to the signal.
Signal conditioning involves reducing the PAPR of modulated signals. Several techniques have been proposed in the literature for this purpose. These include,
1. Various flavors of signal peak clipping.
2. Designing error control codes that lead to signals with reduced PAPR.
3. Designing algorithms to manipulate the data stream from which the modulated signal is derived to yield reduced PAPR.
As explained and discussed above, there are certain disadvantages and shortcomings with respect to the current techniques and approaches that are used to improve PA efficiency. Accordingly, there exists a need for a better method and apparatus for improving power amplifier efficiency in data communication devices and systems such as wireless communication devices and networks which operate according to the Institute of Electrical and Electronic Engineers (IEEE) 802.11 wireless communication standard, published Nov. 16, 1998. The invention described herein fits into the third category of signal conditioning techniques mentioned above.